Our novel gas analyzers operate on the premise that the concentration of a designated gas in a mixture of gases can be measured by passing a particularized beam of infrared radiation through the mixture of gases and ascertaining the attenuation of the energy in a narrow band absorbable by the designated gas with a detector capable of generating an attenuation proportional electrical output signal.
Gas analyzers which similarly employ an infrared source and a detector for generating an electrical signal representing the attenuation of the emitted radiation by a designated gas in the mixture being analyzed have heretofore been proposed. Such devices are commonly referred to as utilizing non-dispersive infrared radiation.
Generation of a detector output signal of a high enough signal-to-noise ratio to be useful requires that the beam of attenuated infrared radiation falling on the detector of such an instrument be modulated. Heretofore, this has perhaps most commonly been accomplished by interposing a spinning wheel between the infrared radiation source and the detector. These wheels, commonly known as choppers, have a series of apertures spaced equally around their peripheries. Consequently, as the wheel rotates, the transmission of the attenuated beam of infrared radiation to the detector of the gas analyzer is alternately enabled and interrupted, typically at a frequency of less than one hundred cycles per second.
Gas analyzers of the character just described are disclosed in U.S. Pat. Nos.: 3,793,525 issued Feb. 19, 1974, to Bursch et al. for DUAL-CELL NON-DISPERSIVE GAS ANALYZER; 4,811,776 issued May 21, 1974, to Blau, Jr. for GAS ANALYZER; 3,987,303 issued Oct. 19, 1976, to Steft et al. for MEDICAL-ANALYTICAL GAS DETECTOR; 4,011,859 issued Mar. 15, 1977, to Frankenberger for METHOD FOR CONTINUOUSLY MEASURING THE CO.sub.2 CONTENT IN BREATHING GAS; 4,204,768 issued May 27, 1980, to N'Guyen for GAS ANALYSERS OF THE SELECTIVE RADIATION ADSORPTION TYPE WITH A CALIBRATION CELL; 4,268,751 issued May 19, 1981, to Fritzlen et al. for INFRARED BREATH ANALYZER; and 4,371,785 issued Feb. 1, 1983, to Pedersen for METHOD AND APPARATUS FOR DETECTION OF FLUIDS and in A Reliable, Accurate CO.sub.2 Analyzer for Medical Use, Solomon, HEWLETT-PACKARD JOURNAL, Sept. 1981, pages 3-21.
Gas analyzers with mechanical choppers such as those described in the just-cited patents have a number of drawbacks. They are bulky, heavy, and expensive; have moving parts, which is undesirable; and also have complex optical designs. They also tend to be less accurate than is desirable and to lack long-term stability.
Also, gas analyzers employing mechanical choppers are relatively fragile. For example, they will typically not work properly, if at all, after they are dropped.
Another heretofore proposed type of gas analyzer employign absorption of infrared radiation as a measure of gas concentration is disclosed in U.S. Pat. No. 3,745,349 issued July 10, 1973, to Liston for SINGLE PATH, DUAL SOURCE RADIANT ENERGY ANALYZER. The infrared radiation source of the Liston device includes wires which are heated by applying pulses of electrical current thereto, thus providing a modulated source of radiation.
While this scheme at least theoretically eliminates the need for a mechanical chopper, it unfortunately has several serious drawbacks. The wires Liston employs are spaced apart. Consequently, the infrared radiation is not uniformly emitted over the area embraced by those wires. As the wires heat up and cool down, their diameters change, affecting the stability of the radiation. The devices cannot be pulsed at frequencies greater than about 20-25 Hz whereas the desirable modulation frequency of the infrared radiation, at least for carbon dioxide, ranges from 40-100 Hz.
Still other patents dealing with pulsed infrared radiation sources are Nos. 3,922,551 issued Nov. 25, 1985, to Williams for DETECTION OF CO.sub.2 IN A HYPERBARIC GASEOUS ENVIRONMENT; 4,084,096 issued Apr. 11, 1976, to Edwards for ELECTRICALLY ACTIVATED INFRARED SOURCE; 4,163,899 issued Aug. 7, 1979, to Burrough for METHOD AND APPARATUS FOR GAS ANALYSIS; and 4,480,190 issued Oct. 30, 1984, to Burrough et al. for NON-DISPERSIVE INFRARED GAS ANALYZER and in a paper entitled Introduction to the State of the Art Gas Sensors by Liston Edwards, Inc., Costa Mesa, Calif.
Williams discloses an infrared radiation emitter employing cyclic variations in gas pressure to modulate the emitted radiation. This scheme is complex and bulky, requires moving parts, and demands a large amount of power.
Burroughs and Burroughs et al. are also concerned with a modulation scheme which employs mechanical parts and requires a large power input. In addition, the scheme disclosed in those patents, which employs an incandescent bulb as a radiation source, is not capable of modulating the emitted radiation to an extent approaching that needed for accurate gas analysis.
Like Liston, Edwards and Liston Edwards employ heated wires as a source of infrared radiation. Their sources therefore have all of the above-discussed drawbacks of Liston's.
An infrared radiation emitter somewhat similar in appearance to those disclosed herein is the subject of U.S. Pat. No. 3,875,413 issued Apr. 1, 1985, to Bridgham for INFRARED RADIATION SOURCE. However, the Bridgham infrared radiation source differs from ours in that he employs a thin film rather than thick film source of infrared radiation. As a result, the Bridgham infrared radiation emitter is not capable of being modulated; and it would be difficult to produce in quantity.